A mixed metal oxide is being used as a positive electrode active material of a secondary battery. Among secondary batteries, a lithium secondary battery has already been put into commercial use as a small power source for cellular phones, notebook computers and the like. Furthermore, because of its applicability as a large power source, for example, as a power source for vehicles such as electric vehicle and hybrid electric vehicle or as a power source for distributed power storages, the demand therefor is on the rise. However, in a lithium secondary battery, a large amount of scarce metal elements such as lithium and cobalt are contained in the raw material of the positive electrode active material, and there is concern about supply of the raw material to meet the growing demand for a large power source.
In response, a sodium secondary battery is being studied as a secondary battery capable of eliminating the concern about supply. The sodium secondary battery can be fabricated using a material which has a plentiful supply and which is inexpensive, and its commercial application is expected to allow for a large supply of large power sources.
In Patent Document 1, a positive electrode active material obtained by calcining a raw material containing Na, Mn and Co in a compositional ratio (Na:Mn:Co) of 0.7:0.5:0.5 is specifically described as the positive electrode active material for sodium secondary batteries.
Patent Document 1: Japanese Unexamined Patent Publication No. 2007-287661 (Example 1)